Near-Surface Site Effects in Crystalline Bedrock: A Comprehensive Analysis of Spectral Amplitudes Determined from a Dense, Three-Component Seismic Array

Abstract

We find that site effects on weathered granodiorite bedrock at Piñon Flat in southern California are highly variable at distance scales of the order of tens of meters. We base this conclusion on 111,632 power spectral estimates obtained from P and S waveforms from 251 local earthquakes recorded in the 1990 Piñon Flat, high-frequency array experiment. Spectral estimates were obtained from each of the 60 three-component stations of this array for each observed P and S phase using the multitaper spectral method. For each event we calculated a median spectrum for the entire surface array from each component and the total power on all three components. We then calculated spectral ratios for each channel and the total power relative to the array median. We found variations of amplitude of these spectral ratios between different stations that exceeded a factor of 10 at some frequencies. Comparison of the entire population of spectral ratios demonstrated that variations seen in the data are unquestionably statistically significant. We display the results using a three-dimensional visualization tool that displays the spectral ratios for the entire array simultaneously, allowing for the observation and interpretation of patterns of spectral variation across the array. By scrolling through frequency we see large variations in spectral amplitude on different components that vary systematically across the array. We also find that the spectral responses of stations in the array systematically differ with event approach azimuth. The scale of these patterns is consistent with Fresnel zone lengths of diffractions induced by scattering within the weathered layer. We compare borehole data to surface data and use synthetic seismograms to show that the high-frequency spectra of the surface stations are extremely sensitive to near-surface velocity structure, as well as surface sensor depth. We suggest that the amplitude variations we see are due to the characteristic, complex, chemical weathering of granitic rocks that leads to a strongly inhomogeneous material. As the seismic wave field encounters this weathered zone, seismic waves are strongly scattered, resulting in spectral variations on short scales both laterally and with depth.